(a) Field of the Invention:
The present invention relates to a method for producing a semiconductor crystal containing Group II-VI compounds. More particularly, the present invention relates to a method for growing a semiconductor crystal of such semiconductor compound as mentioned above through crystallization from a solution thereof.
(b) Description of the Prior Art:
Group II-VI compound semiconductors are of the direct transition type, and are characterized by having a wide energy band gap. Therefore, the Group II-VI compounds are attractive semiconductor materials having desirable unique properties which cannot be obtained in Group III-V compound semiconductor materials. Conduction types and energy band gaps of some of Group II-VI compound crystals are indicated in the following Table.
TABLE ______________________________________ ZnS ZnSe ZnTe CdS CdSe CdTe ______________________________________ Conduction n n p n n n Type p Energy Band 3.6 2.8 2.2 2.5 1.74 1.5 Gap (ev) ______________________________________
Since research has not been made of Group II-VI compound semiconductors so extensively as for the Group III-V compound semiconductors, one has not been able to make full use of the unique properties of the II-VI compound semiconductors as yet. Typical Group II-VI compound semiconductors are ZnSe and ZnS. These compound semiconductors require a more strict control of their vapor pressure in carrying out their crystal growth than that of Group III-V compound semiconductors, because any one of II-VI compound semiconductor crystals has melting points higher than those of the Group III-V compound semiconductor crystals, and both elements which compose Group II-VI compound semiconductors have high vapor pressures. Whereas, in the case of Group III-V compound semiconductors only one element of these two components has a high vapor pressure. For the crystal growth of Group II-VI compound semiconductor crystals, an attempt has been made heretofore to grow a crystal from a melt thereof at a high temperature under a high pressure. In general, grown crystals have a deviation from stoichiometric composition. No counter-method for preventing such deviation from stoichiometry has been taken in the prior art. Another attempt which has been made heretofore is to grow the II-VI compound semiconductor crystal from a solution thereof. Such solution growth method has been most widely employed in the production of Group III-V compound semiconductor crystals which can be grown at a relatively low temperature. The application of the solution growth method to the production of Group II-VI compound semiconductor crystals has not been so extensively developed, and only very few attempts therefor have been reported. This may be attributed to the fact that Group II-VI compound semiconductors have a low solubility in a melt of one of its two component elements, and that both elements have a relatively high vapor pressure, so that it is difficult to carry out the solution growth method which requires the use of a melt of one component serving as a solvent in the growth of Group II-VI compound semiconductor crystal, although it is natural that the solution method is most suitable for growing a crystal containing no extra impurities. For example, in case of compound semiconductor ZnSe, it has a low solubility in a melt of component Zn or Se serving as a solvent, and either component has a relatively high vapor pressure, so that it is difficult to apply the solution growth method where the component is used as a solvent. Alternatively, there has been proposed another type of solution method where Te is used as a solvent for the reasons that Te is an element having a lower vapor pressure among Group VI elements and that ZnSe, ZnS and the like have a higher solubility in the melt of Te. According to this method, however, the resulting crystals are of the type of a mixed crystal containing a several percent of Te and having a ternary composition such as is noted from ZnSe.sub.1-x Te.sub.x, ZnS.sub.1-x Te.sub.x and the like.
For simplicity, the following description is directed to a ZnSe single crystal, and solution growth method. It should be appreciated, however, that this discussion may apply to other Group II-VI compound semiconductor crystals. ZnSe has an energy band gap of 2.8 eV, and will act as a blue color light-emitting diode having a high efficiency, if a p-n junction is formed therein. In case ZnSe contains several percent of Te, the resulting crystal will be a mixed crystal of a composition of ZnSe.sub.1-x Te.sub.x which has a narrower band gap incapable of emitting blue color light, and a tendency to easily generate defects in the crystal due to the presence of irregular strains. Such irregular strains are caused by a large difference in atomic radius between Te and Se. It is desirable, therefore, to obtain a crystal having as low a content of Te as possible so that the crystal may be substantially regarded as ZnSe. However, even when the semiconductor crystal obtained has a composition substantially equal to ZnSe, it has generally a large amount of Se lattice vacancies produced due to a high vapor pressure of Se. The lattice vacancies act as donors. Therefore, only n-type semiconductor crystals have been commonly produced, and no practical p-n junction has been obtained. Moreover, Se lattice vacancies combine impurities to form deep levels which act as non-radiative sites. Even when a p-n junction may be produced, it inevitably has a poor radiation efficiency due to the formation of deep levels. Therefore, there has been a need for a technique which makes it possible to produce semiconductor crystals having high crystal perfection that allows a practical p-n junction to be produced therefrom.